1. Field of the Invention
This invention relates to an apparatus and process for accurate quantitative determination by acoustical methods of insects present in agricultural commodities such as grains, fruits, nuts, vegetables, and legumes.
2. Description of the Prior Art
All grain being exported from this country, as well as most grain being bought for domestic human consumption, is inspected for insect infestation. Presently, inspection of grain is done by counting insects sieved from a defined sample, usually a 1 Kg. sample. This procedure limits detection to externally-feeding larvae and adults. Internally feeding larvae in grain are not detectable by usual methods of analysis, and if adult insects have been removed by cleaning of the grain, infested grain may be declared to be insect free. When these insects emerge days or weeks later, often after the grain has been moved, problems arise that are both legal (where and when did the grain become infested and who is financially responsible) and entomological (movement of insects from this infested grain to nearby uninfected grain creates an insect control problem). Also, larvae in grain kernels are a major source of insect fragments in milled products such as flour which create regulatory and public relations problems for milling companies.
Insects having internally feeding larvae include the lesser grain borer, Rhyzopertha dominica (F.), the rice weevil, Sitophilus oryza (L.), and the Angoumois grain moth, Sitotroga cerealella (Olivier). The eggs of these species are laid either on or in the kernels. Upon hatching, the larvae bore into the kernel and they emerge a few weeks later as adult insects after eating the contents of the grain kernel.
Various approaches to detect larvae in grain have been tried including X-ray analysis of grain (Milner, M., M. R. Lee and R. Katz. [1950]. J. Econ. Entomol. 43: 933-935) and detection of the CO.sub.2 released by respiring insects (Bruce, W. A., M. W. Street, R. C. Semper, and David Fulk, [1982]. Advances in Agric. Tech., South. Series, No. 26, U.S. Dept. Agric. 1-8). These methods are costly, time consuming and generally not implemented. Detection of sounds produced by feeding larvae (Brain, C. K. [1924] Ann. Univ. Stellenbosch. 2: 45-47., Adams R. E., J. E. Wolfe, M. Milner, and J. A. Shellenberger [1953]. Science 118: 163-164, Bailey, S. W., and J. B. McCabe [1965]. J. Stored Prod. Res. 1: 201-212, and Street, M. W. [1971] J. Ga. Entomol. Soc. 6: 72-75) has also been tried but technical difficulties prevented the development of practical systems by later workers. Recent improvements in the sensitivity of acoustical systems for detecting insect larvae in grain suggest that this approach has considerable promise as a fast, simple and an inexpensive method for the practical problems of insect detection (Webb, J. C., C. A. Litzkow, and D. C. Slaughter, [1988]. Appl. Eng. Agric. 4: 268-274, Vick, K. W., J. C. Webb, B. A. Weaver and C. A. Litzkow [1988]. J. Econ. Entomol. 81: 1489-1493, Hagstrum, D. W., J. C. Webb, and K. W. Vick. [1988]Florida Entomol. 71: 441-447, U.S. Pat. Nos. 4,671,114 and 4,937,555).
However, these methods are effective only when the detection problem is simply a yes/no question--are insects present or not, such as in quarantine situations. The situation in grain is complicated by the presence of allowable infestation levels. Only if infestation levels rise above these threshold levels, are penalties accessed. Thus the detection of insects in grain is not a yes/no question, but a quantitative question of whether or not allowable threshold levels are exceeded. How many insects are present? The answer to this question is substantially more difficult to obtain than a yes/no answer, especially at the required accuracy levels. Both Vick et al. (1988), supra and Hagstrum et al. (1988) supra, demonstrated that the number of insect-produced sounds detected in a grain sample or in a grain bin with a grain probe is proportional to the number of insects in the sample or near the grain probe. Calibration curves can thus be produced by experimentally infesting grain at various infestation levels and counting the number of voltage spikes produced by the feeding sounds. Regression lines can be drawn correlating infestation levels to voltage spikes counted so that the numbers of insects in grain samples can be estimated by these voltage spikes. Unfortunately, the number of sounds detected in a grain sample (or with a grain probe in a grain bin) is also affected by the distance of the insect from the detector and the age of the larvae. Grain attenuates the sounds produced by feeding larvae such that an acoustic transducer will detect many more sounds from a larva near the transducer than from one located farther away. For a distant larva, only the strongest sounds are detected, whereas for a larva in close proximity to the transducer, the strongest sounds as well as much weaker sounds are heard. Errors of as much as 100 fold could be made in estimating infestation levels because of the distance effect on sound detection. To circumvent this source of error, grain samples would have to be acoustically analyzed several times with mixing of the sample between analyses in order to achieve an "average" number of sounds detected. The experiments used to collect the data for the calibration curves would have to be collected the same way to avoid introducing "distance" bias into the calibration curves.
Another unwanted correlate to number of sounds detected by the acoustic system is the age of the larvae. As shown by Vick et al. (1988) supra, young larvae produce fewer detectable sounds than older larvae. An extremely large population of very young larvae might produce few detectable sounds and would be mistaken for a small infestation. In most cases the larval population can be assumed to be a randomly mixed age and if the insects used to make the calibration curves were similarly of mixed ages, the error associated with age might largely be canceled out. There are cases where the larval population will not be of uniformly mixed age and in fact can be synchronized. This can happen at the start of a storage season or after a fumigation, for example. In cases of synchronized populations, very large errors can be inadvertently made, both over and underestimating the size of the larval population. Workers estimating larval populations by comparing total insect sounds detected in an unknown sample of grain to a calibration curve would have to be aware of the storage history for each sample that might have synchronized the larval population.
There is a need for a rapid, quantitative, and economic method for detecting larvae of major insect pests of agricultural commodities. The invention described below largely eliminates the problems associated with distance and larval age from the quantitation of population levels in stored agricultural commodities by acoustic means. It also allows the acoustical determination of the number of insects in a sample of grain, fruits, nuts, vegetables or legumes without the need for reference calibration curves.